Search

Carrier dynamics in AlGaN-based single quantum well (QW) structures grown on sapphire are studied by means of time-integrated and time-resolved photoluminescence spectroscopy (PL) in a wide temperature range from 5 K to 350 K. The samples cover a broad compositional range, with aluminum contents ranging between 42% and 60% and QW widths between 1.5 nm and 2.5 nm. All samples reveal the characteristic "S"-shape temperature dependence of the PL emission energy as frequently reported in InGaN-based systems, albeit with significantly larger localization strengths of up to 60 meV. It is shown that in the compositional range investigated, carrier localization is determined primarily by the QW width and, in contrast, exhibits a much weaker dependence on aluminum concentration. By the combination of time-integrated and time-resolved PL measurements, the localization of carriers is demonstrated to have a significant impact on the recombination dynamics of AlGaN/AlN QWs grown on sapphire, heavily affecting the internal quantum efficiency and efficiency droop even in standard LED operation conditions. [ABSTRACT FROM AUTHOR]

We report on a systematic study of the determination of the internal quantum efficiency (IQE) in AlGaN-based multiple-quantum-well (MQW) structures using different optical evaluation methodologies and experimental conditions, in order to derive a standard set of measurement conditions for reliable IQE determination. Several potential sources of error that may distort the IQE obtained by optical measurements are discussed, such as carrier transport effects, excitation conditions failing to fulfill ideal resonance conditions, and morphology issues. A series of nominally identical AlGaN-based MQW structures is grown on an AlGaN layer separated by an AlN interlayer of varying thickness. The MQW structures are studied both by resonant and quasiresonant photoluminescence spectroscopy, and IQEs are determined via different commonly employed methods. The obtained values are shown to be significantly affected by the employed excitation conditions, as well as the evaluation techniques. In addition, growth morphology issues and carrier transport effects need to be considered in the interpretation of the measured data, with the latter being investigated in greater detail. The results emphasize the need for an appropriate choice of both experimental conditions and evaluation methodology in order to extract reliable and comparable IQE values. [ABSTRACT FROM AUTHOR]

The influence of the growth substrate on the internal quantum efficiency (IQE) of deep ultraviolet light‐emitting diodes is studied. Two nominally identical Al‐rich AlGaN/AlN multi‐quantum‐well (MQW) structures grown by metal–organic vapor phase epitaxy (MOVPE) on different substrates are investigated. The first MQW structure is grown on a native AlN substrate, whereas the second one is deposited on an AlN template on sapphire. By the combination of atomic force microscopy (AFM), photoluminescence (PL), and cathodoluminescence (CL) spectroscopy, it is demonstrated that the dislocation‐mediated spiral growth of MQWs on sapphire results in the more efficient localization of carriers. This effect helps to prevent nonradiative carrier recombination at point defects, improving the IQE of the structure. [ABSTRACT FROM AUTHOR]

Herein, the optical properties of aluminum nitride (AlN) epitaxial layers grown on sapphire substrates by metal‐organic chemical vapor deposition (MOCVD) are reported. The structures investigated in this study are grown at highly different degrees of supersaturation in the MOCVD process. In addition, both pulsed and continuous growth conditions are employed and AlN is deposited on nucleation layers favoring different polarities. The samples are investigated by photoluminescence (PL), photoluminescence excitation (PLE), and absorption spectroscopy and are found to vary significantly in absorption and emission characteristics. Two distinct absorption bands in the UV‐C spectral range are observed and examined in greater detail, with either giving rise to a significant absorption coefficient of around 1000 cm−1. The corresponding defect transitions are identified by PL spectroscopy. Combined with secondary‐ion mass spectrometry (SIMS) measurements, these absorption bands are allocated to the incorporation of carbon and oxygen impurities, depending on the applied growth conditions. Furthermore, similarities with other epitaxial growth techniques serving as basis for UV‐C applications are highlighted. These results are highly relevant for a better understanding of absorption issues in AlN templates grown by various deposition techniques. In addition, consequences for the growth of efficient UV‐C devices by MOCVD on sapphire substrates are outlined. [ABSTRACT FROM AUTHOR]

The carrier dynamics of Al‐rich AlGaN/AlN quantum well (QW) structures in the presence of strong carrier localization is reported. Excitation density‐dependent photoluminescence (PL) measurements at low temperatures reveal a clear correlation between the onset of efficiency droop and the broadening of the time‐integrated PL spectra. While the droop onset is heavily impacted by the localization strength, the PL emission broadening is observed almost exclusively on the high energy side of the emission spectrum. Spectrally resolved PL decay transient measurements reveal a strong dependency of the carrier lifetimes on the emission photon energy across the spectrum, consistent with a distribution of localized states, as well as on the temperature, depending on the localization strength of the investigated structure. The characteristic "S"‐shaped temperature dependence of the PL emission energy is shown to be directly correlated to the thermal redistribution of carriers between localized states. Based on these findings, the role of carrier localization in the recombination processes in AlGaN QW structures is underlined and its implications for efficiency droop are discussed. [ABSTRACT FROM AUTHOR]

We investigate carrier localization in Al-rich AlGaN/AlN quantum well (QW) structures. Low temperature time-resolved photoluminescence (PL) experiments reveal a strong variation of the carrier decay times with detection photon energy, suggesting a strong impact of carrier localization, which is found to depend primarily on the QW width. In combination with time-integrated PL measurements and numerical band structure calculations, we are able to provide conclusive evidence that the localization strength in AlGaN-based QW structures is directly coupled to the oscillator strength, providing an explanation for its strong dependence on the QW width. This is further supported by the observation of a strong polarization field dependency of the carrier localization, which excludes excitons and may be explained by the accumulation of electrons close to the QW interface, while holes are independently localized across the QW. We complete our discussion by proposing a model to explain the well-known phenomenon of efficiency droop in accordance with our findings, suggesting delocalization-induced Auger recombination as the responsible loss channel. [ABSTRACT FROM AUTHOR]

The degradation of atomic layer deposited aluminum oxide (ALD-Al2O3) at high temperature and high humidity was investigated. The intrinsic hydroxyl concentration of as-deposited ALD-Al2O3 was evaluated using a temperature dependent deposition study and its impact on degradation behavior was analyzed. In addition, the degradation of ALD-Al2O3 was monitored in situ using a plate capacitor with ALD-Al2O3 as dielectric. A model for the ALD-Al2O3 degradation mechanism was proposed based on the penetration of water molecules into the ALD-Al2O3 and on the formation of aluminum hydroxide. Two parameters, delay-time (time till a change in capacitance occurs) and wetting speed (speed of molecular water penetration into the ALD-Al2O3), were extracted from the capacitance measurements in order to evaluate the dependence of ALD-Al2O3 degradation on temperature and humidity. [ABSTRACT FROM AUTHOR]

The article lists the new members of Phi Alpha Theta including Hannah Alley and Minda Anderson from Agnes Scott College, and Patrick Alfonzo and Elizabeth Atkinson from American University.